Method for forming a bottom spin valve magnetoresistive sensor element

ABSTRACT

A method for forming a bottom spin valve sensor having a synthetic antiferromagnetic pinned (SyAP) layer, antiferromagnetically coupled to a pinning layer, in which one of the layers of the SyAP is formed as a three layer lamination that contains a specularly reflecting oxide layer of FeTaO. The sensor formed according to this method has an extremely high GMR ratio and exhibits good pinning strength.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to processes and materials used in thefabrication of a giant magnetoresistive (GMR) sensor, and morespecifically to the use of a novel nano-oxide layer (NOL) in the pinnedlayer of a bottom spin valve sensor structure to improve its GMR ratio.

2. Description of the Related Art

One of the most commonly used structural configurations of magnetic andnon-magnetic layers in giant magnetoresistive (GMR) read-heads is theso-called spin-valve magnetoresistive (SVMR) structure. In the mostbasic version of the SVMR, two ferromagnetic layers such as CoFe or NiFeare separated by a thin layer of electrically conducting butnon-magnetic material such as Cu. One of the layers has itsmagnetization direction fixed in space or “pinned,” by exchange couplingwith an antiferromagnetic (AFM) layer, usually a layer of MnPt, directlydeposited upon it. The remaining ferromagnetic layer, the unpinned orfree layer, can rotate its magnetization vector in response to smallvariations in external magnetic fields such as are produced by movingmagnetic media, (which variations do not affect the magnetizationdirection of the pinned layer). The rotation of one magnetizationrelative to the other produces changes in the resistance(magnetoresistance) of the three layer structure, the changes beingdependent on the cosine of the angle between the two magnetizationvectors. As a result of these resistance variations, a constant “sense”current sent through the SVMR produces voltage variations across it,which are sensed by external circuitry. This effect of magnetizationdirections on electrical resistance is a result of spin dependentelectron scattering, wherein the orientation of the electronic spins ofthe electrons in the sense current relative to the magnetization of thelayer directly affects their scattering cross-sections and,consequently, the resistance of the magnetic material. An older versionof magnetoresistance was the anisotropic magnetoresistive (AMR) effect,wherein the resistance of a magnetic material was found to depend uponthe angle between its magnetization and the direction of a currentthrough it. The discovery of ways to enhance the magnetoresistive effectby the use of two layers of magnetic material rather than one and by themethods used to form these layers, has led to what is now called thegiant magnetorsistive (GMR) effect. It is this GMR which will be thesubject of the present invention.

The major figure of merit for SVMR performance is its magnetoresistiveratio DR/R, which is a measure of the maximum variation of itsresistance that can be expected in operation. Another factor influencingthe performance of a SVMR is the thinness of its free layer, which iscorrelated to the signal amplitude it can provide for the signals itreads. Naturally, as magnetic recording densities become higher, withdensities up to 70 Gb/in² envisioned in the near future, a very strongsignal will be extremely important. The present invention, however, isdirected at the improvement of the magnetoresistive ratio of a sensor,rather than the thinness of its free layer.

Improvements in the magnetoresistive ratio of a sensor element can beexpected if the electrons in the sense current spend more time withinthe magnetically active portions of the sensor. For example, if thesensor element contains electrically conductive layers which do notdirectly contribute to the magnetoresistive effect (eg. they are notmagnetic), then portions of the sense current may be shunted throughthese layers and not contribute to voltage variations across the sensor.It is now generally well accepted that a major contribution to the GMReffect is the presence of interfaces between various layers of thesensor elements. These interfaces produce specular reflection of theelectrons, effectively removing mean-free-path limitations on electronscattering that would normally be placed on them by the externaldimensions of the sensor. The realization of the importance of internalreflections on the magnetoresistive ratio, has produced great interestin the formation of sensor elements that exploit these interfacialscattering effects. For example, various types of capping layers, seedlayers, buffer layers and nano-oxide layers (NOL) have been proposed asmechanisms for improving magnetorsistive ratios of sensor elements.

Huai et al. (U.S. Pat. No. 6,222,707 B1) teaches a method in which aseed layer is used to provide an improved texture for anantiferromagnetic layer grown upon it. The seed layer allows the growthof improved forms of antiferromagnetic pinning layers in bottom spinvalves (spin valves in which the pinned layer is vertically beneath thefree layer) thereby improving the exchange coupling between the pinningand pinned layers and, consequently, improving the magnetoresistiveratio.

Gill (U.S. Pat. No. 6,122,150) teaches a formation in which a syntheticantiparallel (SyAP) tri-layer is formed of two 20 A layers of Co₉₀Fe₁₀of mutually antiparallel magnetizations, separated by an 8 A layer ofRu. This tri-layer is exchange coupled to an antiferromagnetic pinninglayer of 425 A of NiO. The high resistance of this formation restrictsthe amount of shunted sense current.

Gill (U.S. Pat. No. 6,219,208 B1) teaches the formation of a dual spinvalve sensor having a self-pinned layer rather than a layer pinned by anantiferromagnetic pinning layer, thus eliminating that type of layerfrom the fabrication. The self pinning is accomplished by the magneticfield of the sense current. Because the elimination of the usual pinninglayer also eliminates a source of specular reflection, a specialspecularly reflecting layer is formed over the self-pinned layer.

Gill (U.S. Pat. No. 6,181,534 B1) teaches a method for forming amagnetoresistive spin valve sensor element in which copper and nickeloxide specular relection layers are formed on each other and over a freemagnetic layer.

Pinarbasi (U.S. Pat. No. 6,201,671 B1) teaches the formation of bottomspin valve sensor that employs a TaO seed layer for a NiOantiferromagnetic pinning layer for the purpose of improving themagnetoresistive ratio of the sensor.

Pinarbasi (U.S. Pat. No. 6,208,491 B1) teaches the formation of acapping structure comprising layers of CoFe and Ta or, alternativelyCoFe, Cu and Ta, which improves the magnetoresistive performancesubsequent to long periods of time at high temperatures.

The literature also contains reports of magnetoresistive ratioimprovements as a result of the inclusion of novel materials andstructures in the fabrication of sensors. In this regard, Swagten etal., in “Specular Reflections in Spin Valves Bounded by NiO Layers,”IEEE Transactions on Magnetics, Vol. 34, No. 4, July 1998, pp. 948-953,report on achieving increased electron reflectivity by an insulating NiOlayer that is used to exchange bias a spin valve. Swagten et al., in“Enhanced giant magnetoresistance in spin-valves sandwiched betweeninsulating NiO,” Phys. Rev. B, Vol. 53, No. 14, Apr. 1, 1966 also reporton the enhanced GMR effects obtained when sandwiching Co/Cu/Co andNi₈₀Fe₂₀/Cu/Ni₈₀Fe₂₀ between layers of NiO.

Y. Kamiguchi et al., in “CoFe Specular Spin Valve GMR Head Using NOL inPinned Layer,” Paper DB-01, Digest of Intermagnetic Conference 1999,report on a spin valve structure in which the pinned layer contains anano-oxide layer (NOL) which enhances specular electron scattering.

J. C. S. Kools, et al., in “Magnetic Properties of Specular Spin-ValvesContaining Nano-Oxide Layers,” Paper EB-11, Digest of MMM/Intermag. 2001Conference, p. 262, discusses the specular reflection enhancingproperties of NOL layers used in the free ferromagnetic layers and inthe pinned ferromagnetic layers of spin valve structures usingantiferromagnetic pinning layers.

Y. Huai et al., in “Highly Sensitive Spin-Valve Heads with Specular ThinOxide Capping Layers,” Paper EB-14, Digest of MMM/Intermag. 2001Conference, p. 263, discuss the specular reflection enhancing effects ofthin oxide capping layers used in bottom synthetic specular spin-valvestructures.

The present invention provides a method of improving the GMR ratio of abottom spin-valve structure while maintaining good pinning properties,by the insertion of a novel NOL material layer in its pinned layer.

SUMMARY OF THE INVENTION

It is an object of this invention is to provide a method for forming abottom spin-valve sensor element having a higher GMR ratio than thoseformed by prior art methods, while retaining good pinning properties.

In accord with this object, There is provided a method for forming abottom spin valve sensor element based on a NiCr seed layer, said methodcomprising the formation of a novel, specularly reflecting FeTaOnano-oxide layer within the pinned layer of the sensor element.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the present invention areunderstood within the context of the Description of the PreferredEmbodiments, as set forth below.

The Description of the Preferred Embodiments is understood within thecontext of the accompanying figure, wherein:

FIGS. 1a and 1 b are schematic cross-sectional views of a bottomspin-valve sensor formed in accord with the prior art (1 a) and inaccord with the method of the present invention (1 b).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a method for fabricating a bottomspin-valve sensor element of high magnetoresistive ratio by forming anovel, specularly reflecting nano-oxide layer within its pinned layer.

Referring first to FIG. 1a, there is shown a schematic cross-sectionalview of a typical prior art configuration of a bottom spin-filterelement stack having a synthetic antiferromagnetically pinned (SyAP)layer. Compositionally, said stack has the form:

40 A NiCr/130 A MnPt/15 A CoFe/8 A Ru/20 A CoFe/21 A Cu/20 A CoFe/5 ACu/8 A Ta.

where “A” is angstroms.

As shown in FIG. 1a, the stack comprises an NiCr a seed layer (2) ofapproximately 40 angstroms thickness, an MnPt antiferromagnetic pinninglayer (4) of approximately 130 angstroms thickness, a tri-layerformation of CoFe/Ru/CoFe (6), which is a synthetic pinned layer formedof two CoFe ferromagnetic layers (8) and (10), exchange coupled inmutually antiparallel magnetization directions through a Ru couplinglayer (12) of approximately 8 angstroms thickness. CoFe layer (8) isdenoted the second antiparallel layer, AP2, and is formed to a thicknessof approximately 15 angstroms. CoFe layer (10) is denoted the firstantiparallel layer, AP1, and is formed to a thickness of approximately20 angstroms. The term “antiparallel” in this context refers to themutually antiparallel directions of the magnetizations of the two pinnedlayers that corresponds to the low energy state between AP1 and AP2 whenseparated by a Ru layer of approximately 8 A. The “second” layerreferred to in this context is the one contiguous with theantiferromagnetic pinning layer (4), whereas the “first” layer (10) isthe one contiguous with the subsequently deposited Cu spacer layer (14).Said Cu spacer layer (14) is a non-magnetic spacer layer separating thepinned layer (6) from the free ferromagnetic layer (16), which is a CoFelayer deposited to a thickness of approximately 20 angstroms. A Cu layer(18) of approximate thickness 5 angstroms is formed on the free layerand a Ta layer (20) of approximate thickness 8 angstroms is formed onthe Cu layer. Layers (18) and (20) provide a capping formation for thesensor stack. Referring next to FIG. 1b, there is shown a schematiccross-sectional view of the new structure, compositionally describedbelow, formed in accord with the method of the present invention.

40 A NiCr/130 A MnPt/15 A CoFe/8 A Ru/3 A CoFe/5 A FeTa//Ox//20 ACoFe/21 A Cu/20 A CoFe/5 A Cu/8 A Ta.

As is shown schematically in FIG. 1b, the stack comprises an NiCr (orNiFeCr) seed layer (2) of approximately 40 angstroms thickness on whichis formed an MnPt antiferromagnetic pinning layer (4) of approximately130 angstroms thickness. In accord with the present invention, the SyAPis now formed as five layered laminate (30), comprising a secondantiparallel (AP2). CoFe ferromagnetic layer (6) formed to a thicknessof approximately 15 angstroms, on which is formed a non-magnetic Rucoupling layer (10) of approximately 8 angstroms thickness on which isformed a tri-layered first antiparallel (AP1) layer (15). The AP1 andAP2 terminology is the same as that used in describing FIG. 1a. The AP1layer (15), which in the prior art is a single CoFe layer (see layer(10) in FIG. 1a), is now formed as a tri-layer, comprising a layer ofCoFe (17) of approximately 3 angstroms thickness, to strong magneticcoupling between AP1/AP2 since CoFe/Ru/FeTa coupling is weak, on whichCoFe layer is then formed a specularly reflecting FeTaO layer (19) ofapproximately 5 angstroms thickness, on which is formed a layer of CoFe(21) of approximately 20 angstroms thickness. The FeTaO layer is formedas a deposited FeTa layer which is approximately 95% Fe by atomic weightand approximately 5% Ta by atomic weight, which is subsequently oxidizedin either of the following processes in a PM5 TIM module.

Ox1: 50 sccm O₂ flow rate (0.5 mTorr)×10 sec.

Ox2: 75 sccm O₂ flow rate (0.75 mTorr)×10 sec.

Ox3: 100 sccm O₂ flow rate (1.0 mTorr)×10 sec.

Subsequent to the oxidation of the CoFe layer (21), there is formed anon-magnetic spacer layer (14), which is a layer of Cu formed to athickness of approximately 21 angstroms and which separates the SYAPlayer from the ferromagnetic free layer. That ferromagnetic free layer(16) is then formed on the Cu spacer layer as a layer of CoFe ofapproximately 20 angstroms thickness. Upon said free layer there is thenformed a Cu oxidation barrier layer (18) of approximately 5 angstromsthickness, upon which is then formed a specular scattering layer Talayer (20) of approximately 8 angstroms thickness. Although servingdifferent purposes, the Cu/Ta layer is denoted a capping layer.

Subsequent to the formation of the element as above, the magnetizationsof the various layers are established through a three step thermalanneal in the presence of external magnetic fields as follows:

1: 270° C./1 kOe/10 min., for the free layer, the field beinglongitudinally directed.

2: 270° C./8 kOe/3 hr., for the pinned layer, with the field being inthe transverse direction.

3: 210° C./200 Oe/2 hr., for the free layer, the field beinglongitudinally directed.

Experiments performed on sensor stacks formed in accord with the methodsof the prior art and on sensor stacks formed in accord with the methodof the present invention show a distinct improvement in themagnetoresistive properties of the latter stacks as compared with theformer. Table 1 below compares DR/R and DR for a reference prior artstack (row 1) and stacks formed using the method of the presentinvention for each of the three oxidation processes described above(rows 2,3,4 respectively).

TABLE 1 DR/R(%) DRD(Ohm/sq)NiCr40/MnPt130/CoFe15/Ru8/CoFe20/Cu21/CoFe20/Cu5/Ta8 14.00 2.39NiCr40/MnPt130/CoFe15/Ru8/CoFe3/FeTa5/OX1/CoFe20/Cu21/CoFe2O/Cu5/Ta815.16 2.67NiCr40/MnPt130/CoFe15/Ru8/CoFe3/FeTa5/OX2/CoFe20/Cu21/CoFe2O/Cu5/Ta815.96 2.89NiCr40/MnPt130/CoFe15/Ru8/CoFe3/FeTa5/OX3/CoFe20/Cu21/CoFe2O/Cu5/Ta815.16 2.68

All numerical values in the stack formations above are in angstroms. Itcan be seen from the experimental results that the enhancement of DR/Rand DR is a maximum of 14% and 21% respectively for the OX2 sample.Testing of hysteresis loops for the above samples also shows that theloops of the FeTaO samples are comparable to those of the referencesamples indicating that sensor stacks formed in accord with the methodof the present invention display strong coupling between the pinned andpinning layers.

Experimental and theoretical considerations lead us to conclude that theDR/R and DR improvements are a result of the improved specularreflection of conduction electrons provided by the FeTaO layer in theAP1 layer of the pinned layer.

As is understood by a person skilled in the art, the preferredembodiment of the present invention is illustrative of the presentinvention rather than limiting of the present invention. Revisions andmodifications may be made to methods, materials, structures anddimensions employed in practicing the method of the present invention,while still remaining in accord with the spirit and scope of the presentinvention as defined by the appended claims.

What is claimed is:
 1. A method for forming a bottom spin valvemagnetoresistive sensor element comprising: providing a substrate;forming on the substrate a magnetoresistive-property-enhancing seedlayer; forming on said seed layer a pinning layer of antiferromagneticmaterial; forming on said pinning layer a second antiparallel (AP2)pinned layer of ferromagnetic material; forming on said secondantiparallel (AP2) pinned layer a non-magnetic coupling layer; formingon said non-magnetic coupling layer a first antiparallel (AP1) pinnedlayer, said first antiparallel (AP1) pinned layer comprising a firstferromagnetic layer on which is formed a specular reflection enhancinglayer on which is formed a second ferromagnetic layer; forming on saidsecond ferromagnetic layer a non-magnetic spacer layer; forming on saidnon-magnetic spacer layer a ferromagnetic free layer; forming on saidferromagnetic free layer a double-layer capping layer, said cappinglayer comprising a first layer of non-magnetic material on which isformed a second layer of non-magnetic material; thermally annealing saidsensor element at a prescribed succession of temperatures in thepresence of a corresponding sequence of external magnetic fields,establishing, thereby, magnetizations of said free and said pinnedmagnetic layers.
 2. The method of claim 1 wherein the seed layer is alayer of either NiCr or NiFeCr deposited to a thickness of betweenapproximately 30 and 70 angstroms.
 3. The method of claim 1 wherein theantiferromagnetic pinning layer is a layer of antiferromagnetic materialchosen from the group consisting of MnPt, IrMn, NiMn and MnPtPd.
 4. Themethod of claim 3 wherein the antiferromagnetic pinning layer is a layerof MnPt formed to a thickness of between approximately 80 and 250angstroms.
 5. The method of claim 1 wherein the second antiparallelpinned layer (AP2) is a layer of ferromagnetic material chosen from thegroup consisting of CoFe, NiFe and CoFeNi.
 6. The method of claim 5wherein the second antiparallel pinned layer (AP2) is a layer of CoFeformed to a thickness of between approximately 10 and 25 angstroms. 7.The method of claim 1 wherein the non-magnetic coupling layer is a layerof non-magnetic material chosen from the group consisting of Ru, Rh andRe.
 8. The method of claim 7 wherein the non-magnetic coupling layer isa layer of Ru formed to a thickness of between approximately 3 and 9angstroms.
 9. The method of claim 1 wherein the first ferromagneticlayer of said first antiparallel pinned layer (AP1) is a layer offerromagnetic material chosen from the group consisting of CoFe, NiFeand CoFeNi.
 10. The method of claim 9 wherein the first ferromagneticlayer is a layer of CoFe formed to a thickness of between approximately2 and 10 angstroms.
 11. The method of claim 1 wherein the specularlyreflecting layer is a layer of FeTaO formed to a thickness of betweenapproximately 3 and 10 angstroms.
 12. The method of claim 11 wherein thelayer of FeTaO is formed by an oxidation process applied to a layer ofdeposited FeTa.
 13. The method of claim 12 wherein the layer ofdeposited FeTa is a layer which is approximately 95% by atomic weight ofFe and approximately 5% by atomic weight of Ta.
 14. The method of claim12 wherein the oxidation process is carried out in a PM5 TIM module inwhich there is supplied molecular oxygen at a flow rate of betweenapproximately 5 and 50 sccm, but where approximately 50 sccm ispreferred, a pressure of between approximately 0.05 and 0.5 mTorr, butwhere approximately 0.5 mTorr is preferred, for a time duration ofbetween approximately 9 and 11 seconds, but where approximately 10seconds is preferred.
 15. The method of claim 12 wherein the oxidationprocess is carried out in a PM5 TIM module in which there is suppliedmolecular oxygen at a flow rate of between approximately 10 and 80 sccm,but where approximately 75 sccm is preferred, a pressure of betweenapproximately 0.07 and 0.8 mTorr, but where approximately 0.75 mTorr ispreferred, for a time duration of between approximately 9 and 11seconds, but where approximately 10 seconds is preferred.
 16. The methodof claim 12 wherein the oxidation process is carried out in a PM5 TIMmodule in which there is supplied molecular oxygen at a flow rate ofbetween approximately 10 sccm and 110 sccm, but where approximately 100sccm is preferred, a pressure of between approximately 0.1 mTorr and 1.1mTorr, but where approximately 1.0 mTorr is preferred, for a timeduration of between approximately 9 and 11 seconds, but whereapproximately 10 seconds is preferred.
 17. The method of claim 1 whereinthe second ferromagnetic layer of said first antiparallel pinned layer(AP1) is a layer of ferromagnetic material chosen from the groupconsisting of CoFe, NiFe and CoFeNi.
 18. The method of claim 17 whereinthe second ferromagnetic layer is a layer of CoFe formed to a thicknessof between approximately 10 and 30 angstroms.
 19. The method of claim 17wherein the non-magnetic spacer layer is a layer of Cu of thicknessbetween approximately 8 and 30 angstroms.
 20. The method of claim 1wherein the non-magnetic spacer layer is a layer chosen from the groupconsisting of Cu, Ag and Au.
 21. The method of claim 1 wherein thecapping layer comprises a layer of Cu formed to a thickness of betweenapproximately 3 and 20 angstroms, on which is formed a layer of Taformed to a thickness of between approximately 3 and 30 angstroms. 22.The method of claim 1 wherein the annealing process comprises a firstthermal anneal at a temperature of between approximately 240° and 300°C., but where approximately 270° C. is preferred, in an externallongitudinal magnetic field of between approximately 0.9 and 1.1 kOe,but where approximately 1 kOe is preferred, for a time of betweenapproximately 9 and 11 minutes, but where approximately 10 minutes ispreferred, to magnetize the free layer; followed by a second thermalanneal at a temperature of between approximately 240° and 300° C., butwhere approximately 270° C. is preferred, in an external transversemagnetic field of between approximately 7 and 9 kOe, but whereapproximately 8 kOe is preferred, said field directed transversely tothat of the first thermal anneal, for a time of between approximately2.5 and 3.5 hours, but where approximately 3 hours is preferred, tomagnetize the pinned layer; followed by a third thermal anneal at atemperature of between approximately 190° and 240° C., but whereapproximately 210° C. is preferred, in an external longitudinal magneticfield of between approximately 180 Oe and 220 Oe, but whereapproximately 200 Oe is preferred, in the same direction as that of thefirst anneal, for a time of between approximately 1.5 and 2.5 hours, butwhere approximately 2 hours is preferred, to magnetize the free layer.